Field of the Invention
The invention relates to a moving blade for a turbomachine. The invention relates, furthermore, to a turbomachine with a moving blade.
Moving blades for turbomachines, for example moving blades for high-pressure, medium-pressure or low-pressure part turbines of a steam turbine or gas turbine moving blades for compressors or turbines, are conventionally produced from homogeneous metallic alloys. In this case, in addition to milling methods, casting and forging techniques are also used. The metallic raw material is in this case melted and subsequently rolled as bar stock or forged as a blade blank.
A turbomachine of this type contains an individual rotor or a number of rotors that are disposed one behind the other in the axial direction and around the moving blades of which a gaseous or vaporous flow medium flows during operation. The flow medium in this case exerts on the moving blades a force which gives rise to a torque over the rotor or blade wheel and consequently to the working power output. For this purpose, the moving blades are conventionally disposed on a rotatable shaft of the turbomachine, of which the guide vanes disposed on corresponding guide wheels are disposed on the stationary casing, the casing of the turbomachine, the casing surrounding the shaft so as to form a flow duct.
Whereas, in a compressor, mechanical energy is supplied to the flow medium, in a turbine functioning as a turbomachine mechanical energy is extracted from the flow medium flowing through. In a conventional turbomachine with a shaft rotating during operation and with a stationary casing, the centrifugal force in each moving blade fastened to the shaft generates a tensile load on which is superposed a bending load caused by the flow forces of the flow medium. This results in a critical load at those points in the blade foot and in the shaft at which the bending tensile stress and the tensile stress as a result of centrifugal forces are superposed on one another. Owing to the critical load, there is a limit to the blade height in its radial dimension and consequently to the efficiency of the turbomachine.
In particular, the moving blades of steam turbine low-pressure parts (LP moving blades) are predominantly loaded by centrifugal forces as a result of the rotation of the shaft. The load is therefore directly proportional to the density of the blade material used. Since the densities of the materials used are very similar to that of iron, the load in the case of long LP blades is such that a specific blade length cannot be exceeded. This is important particularly for the higher stages of the LP blading, the radial dimensions of which are limited by the limits of the centrifugal force load. Due to the limited blade length, only a specific outlet cross section can be achieved for the flow medium, so that the flow medium, for example the exhaust steam of a low-pressure part turbine, leaves the turbomachine at a high velocity and consequently with high losses.
Previous solutions to the problem for LP moving blades provide for the use of materials consisting of titanium alloys in the case of very high blade lengths. As compared with alloys based on iron, cobalt or nickel, titanium alloys have a lower density, and therefore, with dimensions otherwise being the same, moving blades consisting of this material are subject to lower stresses than moving blades consisting of the metallic materials customary hitherto. The disadvantage of this solution to the problem is, however, that titanium alloys are very costly and the problem of the centrifugal force load persists, as before, albeit to a somewhat lesser extent.
It is accordingly an object of the invention to provide a moving blade for a turbomachine and a turbomachine which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which specifies a blade configuration that, under the given loads in the turbomachine, does not exceed the permissible stresses and nevertheless allows high efficiency. A further object of the invention is to specify a turbomachine for high stresses, along with high efficiency.
With the foregoing and other objects in view there is provided, in accordance with the invention, a moving blade for a turbomachine. The moving blade has a moving blade body containing, at least in regions, a cellular material and an outer surface. The cellular material has cells forming the outer surface with a structure being closed with respect to the cells.
According to the invention, the object directed at the moving blade is achieved by the moving blade for the turbomachine, the moving blade containing, at least in regions, a cellular material.
As compared with the conventional configurations of moving blades for turbomachines, for example gas or steam turbines, the invention takes a completely new path. Although homogeneous metallic materials have been used hitherto for the moving blades, the concept of the invention is based on the structural configuration of the moving blade and of the materials forming it. By cellular materials being used for the moving blade, a considerable reduction in the average density for the moving blade is achieved. The cellular structure ensures a substantially lower density than homogeneous materials customary hitherto. Since the cellular material is disposed in regions in a specific way, moving blades according to the invention therefore give rise to substantially lower stresses as a result of centrifugal forces. Consequently, when cellular materials are used, moving blades with a markedly higher blade length can be produced, so that a larger flow cross section with lower losses when the moving blade is used in a turbomachine can be implemented.
Moreover, cellular materials have higher internal damping than homogeneous materials, so that they advantageously damp possible vibrations particularly efficiently. Furthermore, cellular materials exhibit good rigidity properties, so that, owing to the high specific strength, they have approximately the permissible load of comparable homogeneous materials. This is particularly advantageous in application in a turbomachine, where considerable thermomechanical loads are to be noted. By virtue of the specific selection of regions of the moving blade where the cellular material is provided, a load-adapted blade configuration can be specified for the moving blade. Depending on the application, therefore, different regions of the moving blade may have the cellular material.
The moving blade preferably has a blade leaf region with the cellular material. It is precisely the blade leaf region of a moving blade which, when the moving blade is used in a turbomachine, is exposed to particularly high blade stresses as result of the action of centrifugal force, since, as compared with other regions of the moving blade, the blade leaf region is at a greater radial distance from the axis of rotation. As a result of the markedly lower density, a blade leaf region having the cellular material undergoes a correspondingly lower centrifugal load.
Preferably, the moving blade has a fastening region, in particular a blade foot, the cellular material being provided in the fastening region. The fastening of a moving blade takes place normally on a rotatable shaft, a fastening region of the moving blade being connected to a corresponding reception region of the shaft. Various blade fastening concepts are known, for example pine tree slot connections or hammer head connections, to which the novel moving blade concept can be applied. By the cellular material being provided in the fastening region of the moving blade, the blade stresses in the fastening region, too, can be reduced correspondingly. By the combination of various regions of the moving blade in which the cellular material is provided, specific adaptation to the respective loads becomes possible. For example, the cellular material may be provided both in the blade leaf region and in the fastening region.
The moving blade may also be formed of as a whole of the cellular material, as a result of which, because of the reduction in density in relation to a comparable solid material, a lightweight form of construction of the moving blade is achieved overall. In terms of the physical properties, such as weight, hardness and flexibility, the cellular construction of the moving blade is far superior to the use of solid light metals, for example titanium alloys.
In a preferred embodiment, the moving blade has an inner region and a casing region surrounding the inner region, the cellular material being provided in the casing region and/or in the inner region.
Also preferably, the cellular material forms an outer surface with a structure that is closed with respect to the cells. This is particularly advantageous, insofar as the outer surface is a part surface of the blade leaf region of the moving blade, the blade leaf region being acted upon by a flow medium during operation. By the outer surface being produced with a closed structure, a surface, for example a surface in the blade leaf region, with correspondingly low roughness is provided. Insofar as the outer surface of the cellular structure is exposed to a flow medium, the flow resistances and consequently the flow losses are correspondingly low. Advantageously, due to the cellular structure of the material, an outer surface is provided which also has a highly damping action with respect to secondary losses as a result of transverse flows. For this purpose, for a possible transverse flow, the surface has barriers that may be formed along mutually contiguous cells of the cellular structure.
In a particularly preferred embodiment, the cellular material is a metal foam. Metal foams, above all, are lightweight construction materials with high potential and with a widespread field of use. Metal foams may be obtained by various production methods, for example by fusion and powder-metallurgic precipitation and sputtering techniques. In a powder-metallurgic method, by a metal powder being mixed with an expanding agent, for example metal hydride, an exchange material is produced, which, after subsequent axial hot pressing or extrusion, is compacted into a prefabricated semi-finished product which, by appropriate forming, can be adapted in a dimensionally accurate manner to a respective final product and, by corresponding heating, is properly foamed to just above the fusion temperature of the metal. The expanding agent which is contained in the semi-finished product, and for which titanium hydride is typically used, decomposes during heating and splits off hydrogen gas. The hydrogen occurring in gaseous form leads as a propellant to forming a corresponding pore formation in the metal melt. The metal foam porosity formed by the pores can in this case be set specifically for the duration of the foaming operation.
Preferably, the density of the metal foam is between about 5% and 50%, in particular between about 8% and 20%, of the density of the solid material.
Preferably, the metal foam consists of a material resistant to high temperature, in particular a nickel-based or cobalt-based alloy. The selection of a material resistant to high temperature is particularly advantageous especially for use in a gas turbine having turbine inlet temperatures of up to 1200xc2x0 C. Use in a steam turbine with high steam states with a steam temperature of more than 600xc2x0 C. is also made possible by the selection of material for the metal foam.
Preferably, the moving blade is configured as a gas turbine moving blade, a steam turbine moving blade, in particular a low-pressure steam turbine moving blade, or a compressor moving blade. In particular, the use of the moving blade in a low-pressure steam turbine appears to be particularly advantageous, because, due to the use of the cellular material, for example the metal foam, higher blade lengths, along with a lower centrifugal force load, can be implemented, as compared with the conventional moving blades. This has a beneficial effect directly on the efficiency of the turbomachine, for example of a low-pressure steam turbine.
The object directed at a turbomachine is achieved, according to the invention, by a turbomachine having a moving blade according to the statements made above.
The turbomachine is advantageously configured as a gas turbine, a steam turbine or a compressor.
The advantages of such a turbomachine may be gathered according to the statements relating to the moving blade.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a moving blade for a turbomachine and turbomachine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.